1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and more particularly to an improved method of forming a field insulating layer which isolates circuit elements therebetween.
2. Description of the Prior Art
In a process for manufacturing a semiconductor device of the prior art, a large height difference is created between the surface of the silicon dioxide layer, formed on a region for isolating semiconductor circuit elements from one another, and the surface of one of the elements. Accordingly, in order to prevent the large height difference from occurring, a so-called Planox process and a LOCOS (Local Oxidation of Silicon) process have been suggested. According to the Planox process, firstly, a region, in which a semiconductor circuit element is to be formed, of a silicon semiconductor substrate is selectively masked with a silicon nitride layer, then, an unmasked region of the substrate is oxidized to form a thick field oxide layer. After that, the mask of the layer of silicon nitride is removed and, finally, in the exposed region a semiconductor circuit element, for example, a transistor element, is formed. The LOCOS process consists of steps similar to the above-mentioned steps in the Planox process. In a semiconductor device manufactured by the Planox and LOCOS processes, a silicon dioxide layer on a region for isolating the circuit elements from one another is formed thick enough that the capacitance between the semiconductor substrate and conductive metal layers to be formed on the thick silicon dioxide layer is decreased. In particular a semiconductor device manufactured by applying the Planox process has a small height difference between the surfaces of the semiconductor substrate and silicon dioxide layer. Thus it is possible to prevent each of the conductive metal layers from breaking.
However, when selective oxidation of the silicon semiconductor substrate is carried out by heating at an elevated temperature in the above mentioned processes, the silicon nitride layer masking the region of the substrate, generates strain in the masked region of the substrate. This strain causes a lattice-defect in the silicon in the region of the substrate where the circuit element is to be formed. Moreover, during the selective oxidation of the silicon substrate, the formed silicon dioxide penetrates deeply under the silicon nitride layer, namely. Thus a so-called bird beak is formed and the region for forming the circuit element becomes narrow.